This invention relates to a synchronous turbine type electric motor. A turbine type electric motor is an alternating current machine designed for high speed operation and having an excitation winding imbedded in slots in a cylindrical steel rotor made from forgings or thin disks. The motor has a stator and a rotor, with the rotor having a large mass of solid iron or iron alloy.
In a synchronous motor there are usually two sets of windings. One of the sets of windings is in the stator, and these windings provides the revolving field that is needed to drive the rotor. The second set of windings is in the rotor and they provide a constant field, so that when the rotor is travelling at near speed of the revolving field, the two fields will lock together and the rotor will be pulled around at the same speed as the revolving field.
A synchronous electric motor is generally started the same way as an induction motor. The field circuit in the rotor is usually short-circuited or closed through an adequate resistance, to hold down the high induced voltage which would otherwise appear at the field terminals; and also to prevent a torque contribution by the induced field current. The pull-in torque developed by a particular motor depends to a considerable extent upon the resistance across the field circuit while it is being started.
When the rotor has accelerated to near synchronous speed, a D.C. current is introduced to the rotor winding. The DC current creates a unidirectional field, which interacts with the revolving field of the stator.
Application of the D.C. current at the proper moment can be done automatically by taking advantage of the motor characteristics. One of these characteristics is that as the motor nears synchronous speeds, the frequency in the rotor circuit approaches zero. The second characteristic is that as the rotor "slips" with respect to the revolving field there are corresponding pulsations in the line current. These conditions can be detected by either a frequency sensitive relay in the field circuit or by a special relay in the line, and the relays can then initiate the switching on of the D.C. field current.
Large synchronous motors are customarily not connected on line, but rather are provided with starting devices such as starter motors coupled to the main machine or separate starting machine sets. Such devices are expensive, however, and require additional maintenance.